In computer graphics, billboards are textured rectangles that are transformed such that they always appear parallel to the view plane. Thus, they are similar to billboards along highways in that they are rotated for best visibility. However, they are different from highway billboards since they are dynamically rotated to always offer best visibility.

The main use of billboards is to replace complex three-dimensional models (e.g. grass, bushes, or even trees) by two-dimensional images. In fact, Unity also uses billboards to render grass. Moreover, billboards are often used to render two-dimensional sprites. In both applications, it is crucial that the billboard is always aligned parallel to the view plane in order to keep up the illusion of a three-dimensional shape although only a two-dimensional image is rendered.

Similarly to Section “Skyboxes”, we can use the default cube object to render a billboard. (For it to work with the default quad, you should change the minus sign to a plus sign in the code. For the default plane, you have to use the z coordinate instead of the y coordinate.) The basic idea is to transform only the origin (0,0,0,1){\displaystyle (0,0,0,1)} of the object space to view space with the standard model-view transformation UNITY_MATRIX_MV. (In homogeneous coordinates all points have a 1 as fourth coordinate; see the discussion in Section “Vertex Transformations”.) View space is just a rotated version of world space with the xy{\displaystyle xy} plane parallel to the view plane as discussed in Section “Vertex Transformations”. Thus, this is the correct space to construct an appropriately rotated billboard. We subtract the x{\displaystyle x} and y{\displaystyle y} object coordinates (vertex.x and vertex.y) from the transformed origin in view coordinates and then transform the result with the projection matrix UNITY_MATRIX_P: